This invention relates to an apparatus for preheating upset end portions of upset steel pipes in advance of quenching, tempering or the like heat treatment of the steel pipes.
The upset steel pipes are those steel pipes which are thicker over certain lengths at the pipe end portions than in the remaining main pipe portion and reserved for machining of screw couplings or the like couplings. When the upset steel pipe is subjected to heat treatment such as quenching or tempering for realizing targeted mechanical strength, more time is involved in elevating the temperature of the thickened upset end portions of the pipe because of the larger thickness of the pipe wall at these end portions, thus resulting in lowered operational efficiency. Particularly, since the effect of tempering of steel depends on the operating temperature and time, such difference in the heating of the main portion and upset portions is not desirable for realizing uniform mechanical properties throughout the length of the upset pipe. With this in view, various systems have been devised for preheating only the upset end portions of the upset steel pipe. According to a known system of this kind, in advance of heating the upset steel pipe in a quenching barrel type heating furnace while at the same time the steel pipe is transferred axially, preheating induction heating coils adapted for preheating the forward end portion of the pipe are arranged in the transverse feed line of the steel pipe. Then, preheating induction heating coils adapted for preheating the rear end portion of the pipe are arranged at a certain distance from the first preheating coils for affording a delay corresponding to a heating time delay between the forward and rear portions at the barrel-type furnace as determined by the pipe length and the feed speed. After the forward and rear upset portions are preheated by these coils with a certain relative delay, the steel pipe is transferred to the axial transfer line for heat treatment. According to another known system, while the upset steel pipe is transferred axially in front of a tunnel furnace, the preheating apparatus is moved simultaneously and in parallel with the pipe for preheating both upset pipe end portions.
However, these known systems suffer from the following deficiencies.
In the former system, since the upset steel pipe itself is moved axially between the preheating devices associated with the respective pipe ends, preheating can not be limited precisely to the targeted upset portions the upset only the upset but the remaining main portion of the pipe may be preheated, thus resulting in deteriorated product quality. Secondly, energy losses may be caused because both pipe ends are not heated simultaneously.
In the latter system, since the preheating device is moved simultaneously with the upset steel pipe in the axial direction of the pipe, the apparatus can not be positioned precisely in response to occasional changes in the pipe length and the main pipe portion may thus be heated in case of changes in the pipe length, equally resulting in deteriorated product quality. In addition, the length of the preheating sections can be changed only with considerable difficulties, when the length of the upset portions is changed.